[Qemu-devel] [PATCH 01/38] exec: Fix memory allocation when memory path names new file

[Markus Armbruster]

Commit 8d31d6b extended file_ram_alloc() to accept file names in
addition to directory names.  Even though it passes O_CREAT to open(),
it actually works only for existing files.  Reproducer adapted from
the commit's qemu-doc.texi update:

    $ qemu-system-x86_64 -object 
memory-backend-file,size=2M,mem-path=/dev/hugepages/my-shmem-file,id=mb1
    qemu-system-x86_64: -object 
memory-backend-file,size=2M,mem-path=/dev/hugepages/my-shmem-file,id=mb1: 
failed to get page size of file /dev/hugepages/my-shmem-file: No such file or 
directory

Rearrange the code to create the file (if necessary) before getting
its page size.

While there, replace "hugepages" by "guest RAM" in error messages,
because host memory backends can be used for purposes other than huge
pages, e.g. /dev/shm/ shared memory.  Help text of -mem-path agrees.

Cc: Paolo Bonzini <address@hidden>
Signed-off-by: Markus Armbruster <address@hidden>
---
 exec.c | 50 +++++++++++++++++++++++++++++++-------------------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/exec.c b/exec.c
index c62c439..81b0063 100644
--- a/exec.c
+++ b/exec.c
@@ -1235,36 +1235,25 @@ static void *file_ram_alloc(RAMBlock *block,
                             const char *path,
                             Error **errp)
 {
+    bool unlink_on_error = false;
     struct stat st;
     char *filename;
     char *sanitized_name;
     char *c;
     void *area;
-    int fd;
+    int ret, fd;
     uint64_t hpagesize;
     Error *local_err = NULL;
 
-    hpagesize = gethugepagesize(path, &local_err);
-    if (local_err) {
-        error_propagate(errp, local_err);
-        goto error;
-    }
-    block->mr->align = hpagesize;
-
-    if (memory < hpagesize) {
-        error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to "
-                   "or larger than huge page size 0x%" PRIx64,
-                   memory, hpagesize);
-        goto error;
-    }
-
     if (kvm_enabled() && !kvm_has_sync_mmu()) {
         error_setg(errp,
                    "host lacks kvm mmu notifiers, -mem-path unsupported");
-        goto error;
+        return NULL;
     }
 
-    if (!stat(path, &st) && S_ISDIR(st.st_mode)) {
+    ret = stat(path, &st);
+    if (!ret && S_ISDIR(st.st_mode)) {
+        /* path names a directory -> create a temporary file there */
         /* Make name safe to use with mkstemp by replacing '/' with '_'. */
         sanitized_name = g_strdup(memory_region_name(block->mr));
         for (c = sanitized_name; *c != '\0'; c++) {
@@ -1282,13 +1271,32 @@ static void *file_ram_alloc(RAMBlock *block,
             unlink(filename);
         }
         g_free(filename);
+    } else if (!ret) {
+        /* path names an existing file -> use it */
+        fd = open(path, O_RDWR);
     } else {
+        /* create a new file */
         fd = open(path, O_RDWR | O_CREAT, 0644);
+        unlink_on_error = true;
     }
 
     if (fd < 0) {
         error_setg_errno(errp, errno,
-                         "unable to create backing store for hugepages");
+                         "unable to create backing store for guest RAM");
+        return NULL;
+    }
+
+    hpagesize = gethugepagesize(path, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        goto error;
+    }
+    block->mr->align = hpagesize;
+
+    if (memory < hpagesize) {
+        error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to "
+                   "or larger than page size 0x%" PRIx64,
+                   memory, hpagesize);
         goto error;
     }
 
@@ -1307,7 +1315,7 @@ static void *file_ram_alloc(RAMBlock *block,
     area = qemu_ram_mmap(fd, memory, hpagesize, block->flags & RAM_SHARED);
     if (area == MAP_FAILED) {
         error_setg_errno(errp, errno,
-                         "unable to map backing store for hugepages");
+                         "unable to map backing store for guest RAM");
         close(fd);
         goto error;
     }
@@ -1320,6 +1328,10 @@ static void *file_ram_alloc(RAMBlock *block,
     return area;
 
 error:
+    if (unlink_on_error) {
+        unlink(path);
+    }
+    close(fd);
     return NULL;
 }
 #endif
-- 
2.4.3